Global request IDs using error stacks

worker.js

// ---- User code:
addEventListener("fetch", event => {
  event.respondWith(handleEvent(event))
})

// ---- Lib code:
const globalStuff = new Map();

async function handleEvent({request}) {
  // Generate a unique ID for this request
  const requestId = crypto.randomUUID();

  // Create a dynamic wrapper function with a custom name:
  const wrapperName = 'REQUEST_ID:' + requestId;
  const _dynamicHandleEvent = (...args) => actualHandleEvent(...args);
  Object.defineProperty(_dynamicHandleEvent, 'name', {value: wrapperName});

  // Save stuff globally, indexed by the current request ID
  globalStuff.set('cache:' + requestId, new Map());
  globalStuff.set('logger:' + requestId, {
    log: (...args) => console.log(request, ...args),
    warn: (...args) => console.warn(request, ...args),
  });

  // Continue normal event handling
  let response;
  try {
    response = await _dynamicHandleEvent();
  } catch(error) {
    response = new Response(error.message, { status: 500 });
  } finally {
    // Cleanup
    globalStuff.delete('cache:' + requestId);
    globalStuff.delete('logger:' + requestId);
  }

  return response
}

function actualHandleEvent() {
  return doDeepStuff();
}

function doDeepStuff() {
  // ...
  const requestId = getCurrentRequestId();

  const myRequestCache = globalStuff.get('cache:' + requestId);
  const promise = myRequestCache.get('myFetch') || fetch('https://example.com');
  myRequestCache.set('myFetch', promise);

  const logger = globalStuff.get('logger:' + requestId);
  logger.log('This works!', requestId);

  // ...
  return new Response('Found Id: ' + requestId, {headers: {'content-type': 'text/plain'}})
}

function getCurrentRequestId() {
  // Error stacks will contain the name of the wrapper function
  const e = new Error('look ma no hands');

  // Using the optional Error.prepareStackTrace below:
  const requestId = e.stack;

  // Parsing the raw stack:
  // const [, requestId] = e.stack.match(/REQUEST_ID:([\w-]+)\s/);

  return requestId;
}


// ---- Optional perf++, avoids creating error stacks.
// From https://v8.dev/docs/stack-trace-api#customizing-stack-traces :
// For efficiency stack traces are not formatted when they are captured but on demand,
// the first time the stack property is accessed.
Error.prepareStackTrace = function (error, stacks) {
  if (error.message === 'look ma no hands') {
    for (let i = stacks.length - 1; i > 0; i--) {
      const fnName = stacks[i].getFunctionName() || '';
      if (fnName.startsWith('REQUEST_ID:')) {
        return fnName.split(':')[1];
      }
    }
  }

  // Fallback for normal errors
  return error.stack;
};